1. Technical Field
The present invention relates generally to refrigerant based heat exchange systems, and more particularly vapor compression refrigeration systems employing a compressor, a condenser, an evaporator and associated fluid circuitry used for air conditioning, food storage refrigeration, and similar applications, and to improvements which result from subcooling the refrigerant prior to it reaching the evaporator.
2. Description of the Related Art
The use of refrigerated air conditioning systems in commercial and residential property has become commonplace and ubiquitous. Indeed, particularly in the South and Southwest, it borders on being a necessity for ordinary life. Over the years, a variety of different air conditioning systems have been developed for cooling interior spaces. For example, in particularly arid regions, evaporative coolers are effective air conditioners, while large commercial buildings oftentimes rely upon air conditioning systems commonly known as chilled-water systems. Perhaps the most widely employed air conditioning system used today is what is commonly termed refrigerated air.
Refrigerated air conditioning systems utilize a thermal transfer cycle commonly referred to as the vapor-compression refrigeration cycle. Such systems typically include a compressor, a condenser, an expansion or throttling device and an evaporator connected in serial fluid communication with one another forming an air conditioning or refrigeration circuit. The system is charged with a condensable refrigerant (e.g., R-22 or R-410A), which circulates through each of the components in a closed loop. More particularly, the refrigerant of the system circulates through each of the components to remove heat from the evaporator and transfer heat to the condenser. The compressor compresses the refrigerant from a low-pressure superheated vapor state to a high pressure superheated vapor thereby increasing the temperature, enthalpy and pressure of the refrigerant. A superheated vapor is a vapor that has been heated above its boiling point temperature. The refrigerant then leaves the compressor and enters the condenser as a vapor at some elevated pressure where it is condensed as a result of heat transfer to cooling water and/or ambient air. The refrigerant then flows through the condenser condensing the refrigerant at a substantially constant pressure to a saturated-liquid state. The refrigerant then leaves the condenser as a high pressure liquid. The pressure of the liquid is decreased as it flows through the expansion or throttling valve causing the refrigerant to change to a mixed liquid-vapor state. The remaining liquid, now at low pressure, is vaporized in the evaporator as a result of heat transfer from the refrigerated space. This low-pressure superheated vapor refrigerant then enters the compressor to complete the cycle.
While all refrigerated air conditioning systems operate in accordance with the same general principals, there are a multitude of specific configurations adapted to particular uses. With regard to residential and smaller commercial building applications, one system in particular, commonly known as the “split-system,” has become quite prevalent. As its name implies, split-system air conditioners split the “hot” side from the “cold” side of the vapor-compression refrigeration cycle. The hot side of the system, known as the condensing unit, is placed outside the building and comprises a compressor, a condenser heat exchange coil and a fan to disperse the heat generated by the system. The cold side of the system, comprising an expansion valve and evaporator coil, is generally placed in an air handler unit, such as the furnace or some other air circulating device on the interior of the building. The air handler unit blows air over the evaporator coil and routes the air throughout the building using a series of ducts. Because the two major components of a split-system air conditioner are remotely located from one another, connecting lines are used to link the two components together.
For example, FIG. 1 depicts a typical split-system design of a vapor-compression refrigeration cycle used in residential applications. A house 10 is shown having an air handler unit, designated generally as 12, located on the interior of house 10 for directing conditioned air throughout the house by way of air ducts (not shown). Located within the air handler unit 12 is a throttling or expansion valve in fluid communication with an evaporator coil for evaporating the refrigerant to cool the evaporator coil so that when air is directed over the evaporator coil, the air is cooled and then distributed by the air handler around the house. A condenser unit 20, remote from air handler 12, is located outside house 10 and includes a compressor 14, a condenser coil 22, and an exhaust fan 18 housed in a protective housing. For example, as depicted in FIG. 1, the housing includes a top 21 and corners 23 constructed of sheet metal and protective wire grates 24. The exhaust fan 18 is typically powered by an electric motor 16. The exhaust fan 18 is used to create an air flow over the condenser coil 22 thereby cooling and condensing the compressed refrigerant vapor into liquid refrigerant. The condenser coil 22 typically includes a plurality of parallel planar heat transfer fins 25 to assist the heat transfer from the condenser coil to the air flow.
Thus, in accordance with a typical vapor-compression refrigeration cycle, low-pressure refrigerant vapor is compressed by the compressor 14 and fed to the condenser coil 22 where the high pressure superheated refrigerant vapor releases heat to the airflow generated by exhaust fan 18 and condenses. The high pressure condensed refrigerant then exits the condenser unit 20 and flows through a conduit 2 to the throttling or expansion valve where the pressure of the refrigerant is reduced. From the expansion device, the refrigerant passes into the evaporator coil, absorbs ambient heat from air directed over the evaporator, and vaporizes. The low-pressure superheated refrigerant vapor is then drawn back into the compressor 14 by means of return conduit 4, completing the circuit.
It is, of course, understood that the system shown in FIG. 1 is a simplified depiction of the basic components in a typical vapor-compression refrigeration cycle. Indeed, the great majority of innovations in the field of such refrigeration systems have been directed at improving the individual components of the cycle. For example, while the condenser coil 22 is depicted as a single helically wound vertically disposed coil, it is understood that, in accordance with known prior art, the condenser coil 22 may comprise a plurality of refrigerant circuits, wherein each refrigerant circuit comprises a plurality of refrigerant tubes which run transverse to the fin structure of the heat exchanger, with the ends of appropriate tubes in each circuit being connected by curved tubes or return bends. Each refrigerant circuit receives a portion of the high pressure superheated refrigerant vapor produced by the compressor 14 through a distributor valve (not shown). A collection manifold (not shown), in turn, receives the condensed refrigerant from each of the circuits, where it combines to flow through conduit 2 to the throttling or expansion valve. Likewise, compressor 14 may comprise a variable capacity compressor and electric motor 16 may include variable speed drives.
The efficient operation of refrigerated air conditioners is of continuing and ever increasing importance as energy costs continue to rise. A variety of other proposals have previously been made to improve the efficiency of refrigeration systems featuring the vapor-compression refrigeration cycle. For example, many proposals have sought to improve the heat transfer characteristics of the condensing coil. Two simple solutions include increasing the mass of the condensing coil or increasing the air flow over the condensing coil. However, both of these solutions are not economically efficient in view of present costs of materials and energy. Other attempts to provide increased efficiency have resulted in various designs for applying water to the condensing coil to improve its heat transfer characteristics and to further cool the liquid refrigerant prior to evaporation. For example, Gray U.S. Pat. No. 6,761,039, issued Jul. 13, 2004, discloses such a cooling system for spraying water onto the condensing coil.
In addition, it is also known that the subcooling of a liquid refrigerant below the temperature at which it was condensed will increase the refrigerating capacity of the compressor system due to less flash gas being produced at the expansion valve by the colder refrigerant and the increased capacity of the refrigerant for absorbing heat.
The subcooling of the liquid refrigerant is an effective means for increasing the refrigeration capacity of a given refrigeration unit. It has been found that for every 2° F. of subcooling of conventional halogenated refrigerants that takes place, the capacity of the refrigerating system will be increased by approximately one percent.
Subcooling the liquid refrigerant on the downstream or liquid side of the condenser thus holds promise if the increase in efficiency is improved to make it economically feasible. The effect of this subcooling can be visualized on the standard pressure/enthalpy chart for the standard CFC refrigerant. The cooling capacity of the refrigerant is increased as represented by the increased area on the left side within the diagram lines of the chart. The saturated liquid refrigerant is cooled beyond the reference line on the left side providing an increase in efficiency.
There have been some efforts in the prior art to use auxiliary cooling devices as subcoolers. In this effort for example, additional heat exchange coils are provided in the closed loop refrigeration system downstream of the condenser. This art typically comprises expensive and complicated counterflow heat exchanger type add-on or retrofit for existing refrigeration systems or the like. A typical system utilizing a counterflow liquid cooling coil is shown in Jennings U.S. Pat. No. 3,177,929, issued Apr. 13, 1965. While these units have been around for years, it is generally accepted that they have not been successful because the increase in efficiency of the subcooling unit working alone does not justify the cost of the unit.
A need, therefore, exists for an improved and simplified accessory sub-cooling unit and method of operation which can be easily adapted to an existing condenser unit in a refrigerated air conditioning system utilizing a vapor-compression refrigeration cycle.